1. Field of the Invention
This invention relates to a motor drive system and more specifically, relates to a motor drive system for starting a sensorless motor where the motor rotor may be rotating prior to activation of the motor drive system.
2. Description of the Art
Many motor drive systems and motors, such as permanent magnet synchronous motors (PMSM), use sensorless control techniques to reduce cost and to increase reliability. These motor drive systems and motors are sensorless in that they do not include functionality to measure the voltage feedback from the motor and/or sensors to detect the position of the motor rotor. Rather, rotor position is determined based on estimates of the motor winding currents.
Notably, due to an external load, the rotor of a motor may be rotating (forward or reverse) prior to activation of the motor drive inverter. For example, if a sensorless motor drive system and motor are for an outdoor fan (such as for an air conditioning unit), the wind may force the fan and thereby the motor rotor to rotate prior to activation of the inverter. As is known, once the rotor is spinning, the motor generates a back EMF. Because the motor drive system and motor are sensorless, however, the phase and magnitude of this back EMF are unknown to the motor drive system when activating the inverter. As a result, when the inverter is activated, the inverter output voltage will typically be out of phase and magnitude with the motor back EMF. Problematically, if the motor is spinning at high enough rate and thereby generating a large back EMF, this mismatch can result in a large motor current, thereby causing over current shutdown of the inverter.
An additional problem also occurs when the rotor has reverse rotation prior to activating the inverter. Under these conditions, the sensorless (i.e., no voltage or Hall sensors, only current sensors) motor drive system must control the motor speed to go from the reverse direction to the forward direction, which may be difficult when the inverter has no regeneration capability and the rotor is spinning at a high rate.
Accordingly, it would be desirable to provide a motor drive system that uses sensorless control techniques and that is able to robustly start a motor when the motor rotor may be rotating in a forward or reverse direction prior to activation of the motor drive inverter.